Plant Tropic Responses
Tropisms are directional movement responses that occur in response to a directional stimulus. Plants are not able to relocated if they happen to start growing where conditions are suboptimal. However, plants can alter their growth so they can grow into more favorable conditions, To do so requires the ability to detect where the conditions are better and then alter their growth so they can "move" in the appropriate direction. One of the most commonly observed tropic responses in plants is phototropism, in which plant stems grow towards light. As anyone who has grown plants near a window knows, the plants tend to lean towards the window where the light is usually stronger than inside the room. Another commonly observed tropic responses is gravitropism, where a plant will grow so that it stays oriented relative to the source of gravity (the earth). Thus, if a plant is knocked down the shoot will grow faster on the lower side until the shoot is more-or-less standing up again.
Tropic responses result from differential growth. Phototropism is a blue-light-dependent response controlled by the action of specific blue light photoreceptors called phototropins. Gravitropism is dependent on the presence of starch-filled plastids (amyloplasts) in specialized cells. When the orientation of the cells changes, the mass of the starch-filled plastids causes them to sink to the lower end of the cell. The tumbling of the amyloplasts triggers, through unknown mechanisms, differential growth that causes curvature to develop.
The time-lapse movies available here demonstrate phototropic and gravitropic responses in several different plants. You may see some of the movies already under other subject heading since they show more than just the tropic response (e.g. germinating corn seedlings).